Rare earth metal-based permanent magnets, for instance, R—Fe—B based permanent magnets represented by a Nd—Fe—B based permanent magnet, or R—Fe—N based permanent magnets represented by a Sm—Fe—N based permanent magnet, etc., utilize inexpensive materials abundant in resources and possess superior magnetic characteristics; particularly among them, the R—Fe—B based permanent magnets are employed today in various fields. However, since rare earth metal-based permanent magnets contain a highly reactive rare earth metal: R, they are apt to be oxidized and corroded in ambient, and in case they are used without applying any surface treatment, corrosion tends to proceed from the surface in the presence of small acidic or alkaline substance or water to generate rust, and this brings about the degradation and the fluctuation of magnetic characteristics. Moreover, in case such a rusty magnet is embedded in a magnetic circuit and a like device, there is fear of scattering rust as to contaminate peripheral components. In the light of such circumstances, there has been employed a method for forming a copper plating film, which is a film having superior corrosion resistance, on the surface of the rare earth metal-based permanent magnet.
In general, methods for forming copper plating films are roughly classified into a copper electroplating treatment and a copper electrolessplating treatment; however, it is important to control the plating solution in case a copper plating film is formed on the surface of the rare earth metal-based permanent magnet by means of a copper electrolessplating treatment so as to prevent problems from occurring, because rare earth metals and iron, which are the metal constituents of the magnet, elute out into the plating solution and react with the reducing agent in the plating solution, and the formation of copper plating films proceeds on the surface of the rare earth metals and iron eluted out into the plating solution. However, this is not always easy to put into practice. Furthermore, the plating solution for use in a copper electrolessplating treatment is generally expensive. Accordingly, in case of forming a copper plating film on the surface of a rare earth metal-based permanent magnet, in general, a simple and low cost copper electroplating treatment is employed.
In case of forming a copper plating film on the surface of a rare earth metal-based permanent magnet by means of a copper electroplating treatment, an alkaline plating solution is preferred to be used by taking into consideration of the strong corrosive properties under acidic conditions on the rare earth metal-based permanent magnet. Accordingly, in general, a plating solution containing copper cyanide (copper cyanide plating bath) had been used. However, although copper cyanide plating bath has high utility value considering that it provides a copper plating film having excellent properties and is an easily controllable plating solution, its environmental impact is not negligible because it contains highly toxic cyan. Thus, recently, a plating solution containing copper pyrophosphate (copper pyrophosphate plating bath) is being used more frequently in the place of copper cyanide plating bath; however, since copper pyrophosphate plating bath contains large amount of free copper ions, in case an attempt is made to form a copper plating film directly on the surface of the rare earth metal-based permanent magnet by using copper pyrophosphate plating bath, substitution plating reaction occurs between an electrically base metal constituting the surface of the magnet, i.e., iron and the like, and copper which is an electrically noble metal, thereby causing substitution precipitation of copper on the surface of the magnet. Such factors affect the formation of a copper plating film having excellent adhesiveness, which is found problematic.
In the light of such circumstances, the present inventor has proposed in patent literature 1 a method for forming a copper plating film on the surface of a rare earth metal-based permanent magnet, which comprises carrying out a copper electroplating treatment by using a plating solution having its pH adjusted to a range from 11.0 to 13.0 and containing 0.03 mol/L to 0.5 mol/L of copper sulfate, 0.05 mol/L to 0.7 mol/L of ethylenediamine tetraacetic acid, 0.02 mol/L to 1.0 mol/L of sodium sulfate, and 0.1 mol/L to 1.0 mol/L of at least one type selected from tartarates and citrates. According to this method, a copper plating film having extremely superior adhesiveness can be formed on the surface of a rare earth metal-based permanent magnet, as compared with the case of applying a copper electroplating treatment by using copper pyrophosphate plating bath. However, even with this method, it was found still unfeasible to form a copper plating film on the surface of a rare earth metal-based permanent magnet, which assures sufficiently high adhesiveness for the corrosion resistance necessary for a rare earth metal-based permanent magnet used under severe environment.
In such a case, the adhesiveness of a copper plating film can be compensated by a method, as disclosed in patent literature 1, which comprises forming a nickel strike plating film on the surface of the rare earth metal-based permanent magnet, and then, forming a copper plating film (with regard to a method for forming a nickel strike plating film on the surface of a rare earth metal-based permanent magnet, reference can be made to, for instance, patent literature 2). This method enables forming a laminated film having extremely superior adhesiveness on the surface of a rare earth metal-based permanent magnet, however, a nickel plating film is apt to co-precipitate hydrogen during the electroplating process. Hence, in case of forming a nickel strike plating film on the surface of the rare earth metal-based permanent magnet, there is fear of causing embrittlement of the magnet due to the co-precipitated hydrogen, which leads to the degradation of magnetic characteristics of the magnet. Thus, the development of a novel method capable of forming directly a copper plating film having excellent adhesiveness on the surface of a rare earth metal-based permanent magnet by means of a copper electroplating treatment is keenly demanded.
Under such circumstances, in patent literature 3 is proposed “a surface treatment method for magnets, characterized by forming a first protective film comprising a copper film on the surface of a magnet containing rare earth metals, by electroplating with the use of a copper plating solution containing at least a copper salt compound, a phosphorus compound, an aliphatic phosphonic acid compound, and a hydroxide”, as a method for forming a copper plating film having excellent adhesiveness on the surface of a rare earth metal-based permanent magnet by means of a copper electroplating treatment. However, concerning the aliphatic phosphonic acid compound, which is the constituent component of the plating solution, patent literature 3 only mentions a phosphonic acid alkali metal compound, a phosphonic acid transition metal compound, and the like, as examples; which reference can be made to paragraph number 0039 in the description thereof, but since no specific compounds are exemplified, regretfully, the actual process cannot be understood.    Patent Literature 1: JP-A-2004-137533    Patent Literature 2: JP-A-6-13218    Patent Literature 3: JP-A-2001-295091